Appendix A — Dimensions of matrices

When extracting an entire row or column using [ ], the object that R returns is a vector rather than a matrix. This means that we can’t use some functions that only work for matrices (or arrays).

For example, the code below returns the value NULL when using the dim() function. dim() should return the dimensions of an array, but since the extracted row for births in Edinburgh is a vector, there are no dimensions to return.

R Code

dim(births["Edinburgh", ])

Output

NULL

If we want to know how many elements are in a vector, we use the function length().

R Code

length(births["Edinburgh", ])

Output

[1] 3

We can force the output from subseting a matrix/array to be a matrix/array by including a third argument, drop = FALSE, within the square brackets to keep the returned row as a matrix/array.

R Code

dim(births["Edinburgh", , drop = FALSE])

Output

[1] 1 3

Now we can see that the row is seen as a 1 \(\times\) 3 matrix by R.